Any ply that carries a BS 1088 or 6566 stamp is made to a very demanding set of standards. Most of the kit makers use it. If it doesn't pass a battery of tests it can't carry the BS stamp. None of this stuff is made in the US.
US plywood is made to standards set by the regional plywood associations. Each association gives a collective sigh when one of it's members violates the association's standards.
If BS ply is out of the question it's best that you evaluate every sheet that you buy.
It should have exterior glue. The panel stamp will say "exposure 2" or "exterior". If it's says "rated sheeting" that's interior but rated for limited exterior exposure. Avoid this stuff.
It should have verneers that are about equal in thickness. This is an important strength issue. Ply that has a thick core covered by paper thin outer verneers is weaker and won't take much banging around. All the luan hulls I've seen were covered with 'glass and epoxy. $8 a sheet doesn't mean much if you have to cover it with $60 worth of 'glass and epoxy and then lug around all that additional weight. Luan also has the disadvantage of tearout if you use wire to pull obstinate panels together.
I may be wrong but I don't think there is any American made exterior ply that's thinner than 1/4 in. The 1/8 in. ply panels that some call "doorskins" are "exposure 1" interior.
Plenty of boats have been made with interior ply, flakeboard,OSB and probably even masonite. But if water penetrates the finish of any of these there will be serious maintainence issues.
I built a piroge out of an underlayment called "Multiply" by Weyerhauser. It's a hardwood underlayment with all the good qualities. Exterior glue,equal thickness verneers and almost no voids. BUT, it only comes in half-sheets. I don't recommend this approach but it did produce one tough hull.
Hope this helps.
Charlie

: I've built three kit boats and had a blast. I would like to build a S&G from
: plans but having to mail order everything definitely drives the price up.
: Using cheaper wood helps but I'm unsure what to try. The thinnest plywood
: I can locate in my area is 1/4" luan (actually about 3/16") and
: 1/8" birch (which looks pretty good). I think the luan is too thick
: to flex enough in building. Has anyone used 1/8" birch in building?
: If so, please share your experience. Thanks.